I checked default EGPH now, and I can see all jetways are from GSX, in glass, with RBS or Edinburgh logos.
It's strange you see GSX replacement jetways on other default airports, but not there because, if this was a scenery priority problem, since all GSX replacement jetway files are all in the same package, they should be either all visible, or none of them should, this because also all default airports are in the same package so, either the priority between them is correct, or it's wrong for all airports.
Could be you had another non-default airport for EGPH, so GSX disabled it from jetway replacement ( as it should do ), but you don't have it anymore, so you need to enabled it back ?
Check the GSX Config page in the FSDT Installer, and be sure EGPH is not included in the "Disabled" list, which is the normal case if you are using the default airport.